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Introduction
This document is aimed at system administrators of email servers. It is not intended for the general end user. If you are an end user that has nothing to do with the administration of your email server, please forward the error message referring to this page, to your IT department and/or mail server administrator with the message that this is their problem.

Explanation of the 'Host not found' Error
The error message is generated when ever your mail server attempts to send mail to a system that is enforcing RFC 2821 Section 3.6, where a connecting host uses the EHLO command to identify itself, and where there is no corresponding DNS record for that host. You will note that the requirement is that a DNS RR of type A is required, and that there is no requirement for the A record to match the client connecting IP address (As per RFC 1123 Section 5.2.5).

How to fix the 'Host not found' Error
Fixing this issue should be simple. If you administer your own DNS servers simply, add an A record for the hostname that your mail server uses to identify itself (usually its hostname). If you do not administer your own DNS, ask the people/person that does to add an A record for the same host.

Explanation of the 'Invalid name' Error
The error message is generated when ever your mail server attempts to send mail to a system that is enforcing RFC 2821 Section 4.1.1.1 , where a connecting host uses the EHLO command to identify itself and where the hostname contains characters that are not one of the following:

           "a-z", "A-Z", "0-9", "." and "-"

Further the hostname should start with a letter of the alphabet.

How to fix the 'Invalid name' Error
Fixing this issue should be simple. Change the hostname of the machine to one which is only using the characters in the previous section. With some mail servers it is possible to specify the host used by the EHLO command, in which case you may want /have to change this parameter to change the hostname instead of changing the DNS.

Note: When changing the hostname you should add the new hostname to the DNS or you will fall foul of the other checks implemented (See: Explanation of the 'Host not found' Error)

Explanation of the 'need fully-qualified hostname' Error
The error message is generated when ever your mail server attempts to send mail to a system that is enforcing RFC 2821 Section 3.6 , where a connecting host uses the EHLO command to identify itself and where the hostname does not contain a domain part.

How to fix the 'need fully-qualified hostname' Error
Fixing this issue should be simple. The problem is usually cause where the hostname of the machine has not been fully qualified in the operating system. Adding the domain to the hostname of the machine and/or adding the domain information to /etc/resolv.conf (UNIX servers) is often the solution. If you are using an operating system that appears to already know its domain and the mail server is still using the local hostname, check the documentation with reference to overriding the presented hostname with a custom hostname. If you are still unable to resolve the problem, contact your mail server vendor for further assistance.

Why should you fix the Errors?
Spammers and Viruses usually will not use a valid hostname, they often create a random string to identify the recipient should it be submitted as a spam-trap. Further the RFCs are about cooperative implementation of standard protocols. By violating the RFCs you are indicating you do not wish to cooperate with others with respect to exchanging data, so "we" are indicating that by not cooperating you are not permitted to exchange data with "our;" servers.

 
   
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